Electron beam tubes are used for the amplification of RF signals and are typically linear beam devices. There are various types of linear electron beam tube known to those skilled in the art, examples of which are the Klystron, and the Inductive Output Tube (IOT) and Travelling Wave Tubes (TWTs). Linear electron beam tubes incorporate an electron gun for the generation of an electron beam of an appropriate power. The electron gun includes a cathode heated to a high temperature so that the application of an electric field between the cathode and an anode results in the emission of electrons. Typically, the anode is held at ground potential and the cathode at a large negative potential of the order of tens of kilovolts.
Inductive Output Tubes used as amplifiers broadly comprise three sections. An electron gun generates an electron beam, which is modulated by application of an input signal. The electron beam then passes into a second section known as the interaction region, which is surrounded by a cavity arrangement including an output cavity arrangement from which the amplified signal is extracted. The third stage is a collector, which collects the spent electron beam.
In an inductive output tube (IOT) a grid is placed close to and in front of the cathode, and the RF signal to be amplified is applied between the cathode and the grid so that the electron beam generated in the gun is density modulated. The density modulated electron beam is directed through an RF interaction region, which includes one or more resonant cavities, including an output cavity arrangement. The beam may be focused by a magnetic means to ensure that it passes through the RF region and delivers power at an output section within the interaction region where the amplified RF signal is extracted. After passing through the output section, the beam enters the collector where it is collected and the remaining power is dissipated. The amount of power which needs to be dissipated depends upon the efficiency of the linear beam tube, this being the difference between the power of the beam generated at the electron gun region and the RF power extracted in the output coupling of the RF region. The power that is not recovered as electrical energy in the collector creates heating of the collector electrodes. This heat needs to be removed using a cooling arrangement.
The difference between an IOT and a klystron is that in an IOT, the RF input signal is applied between a cathode and a grid close to the front of the cathode. This causes density modulation of the electron beam. In contrast, a klystron velocity modulates an electron beam, which then enters a drift space in which electrons that have been speeded up catch up with electrons that have been slowed down. The bunches are thus formed in the drift space, rather than in the gun region itself.
In IOTs, klystrons and other linear beam tube types such as TWTs, the efficiency of collection of the electron beam can be improved by using a multi-stage depressed collector. In such an arrangement, there is a plurality of electrically isolated stages of electrodes, each operating at a potential at or between ground and the cathode potential. In one such typical arrangement, a collector has five stages, the difference in potential between the various stages being 25% of the beam voltage. By using such a multi-stage depressed collector, the electrons in the beam are slowed down before impacting on the electrode surfaces, thus leading to greater recovery of energy. Collectors may, of course, have a different number of stages operating at different potentials. The term “depressed” is used in the sense that the voltage at which each electrode is held is “depressed” in relation to ground potential.
In collectors for electron beam tubes, whether klystron, IOT or other, there is a need for an efficient means of extracting and dissipating heat generated by the electron beam striking the electrode(s) of the collector. This requirement exists for both single stage and multi-stage collectors.
Various cooling techniques are known, broadly falling into three categories: air, oil and water-cooled, each having advantages and disadvantages. An example of an oil-cooled collector is known in WO 00/63944. In this arrangement, the electrically conductive electrodes of the collector are formed with channels on their outer surface and are encased by an electrically and thermally non-conductive inner sleeve to define enclosed channels through which oil is pumped as a coolant. The inner sleeve is surrounded by an electrically and thermally conductive (metal) outer sleeve defining a channel, which communicates with channels of the collector electrodes. Cooling is thereby achieved by contact of the coolant fluid with the electrode stages and so, as the electrodes are at different potentials, the coolant (oil) must be an electrical non-conductor.
A second cooling arrangement is known in U.S. Pat. No. 5,493,178. In this arrangement an electrically non-conductive but thermally conductive body surrounds, and is in contact with, the electrodes of the collector. Coolant channels are provided on the exterior of the thermally conductive body and are enclosed by an outer electrically and thermally conductive (metal) casing. The cooling is thus achieved by thermal conduction through the thermally conductive body to the coolant channels containing a cooling fluid. In this arrangement, the coolant fluid is electrically insulated from the electrodes and so the coolant itself could be electrically conductive, such as normal water.
We have appreciated deficiencies in known designs and appreciated the need to provide good thermal conduction from a collector whilst providing a high level of electrical insulation. We have further appreciated the need to provide resilience to expansion and contraction as the collector heats and cools.
The invention is defined in the claims to which reference is now directed. The preferred embodiments of the invention combine the benefits of both oil and water-cooling by providing oil in contact with electrode(s) and a surrounding water-cooling system separated, at least in part, by a plurality of electrically insulative, thermally conductive solid dielectric spacers. Various configurations of the spacers such as in the form of panels are provided, in embodiments of the invention, in thermal contact with the electrode(s), to provide heat transfer to a coolant whilst providing electrical insulation.
Whilst the preferred choice in each of the embodiments is to use oil as the electrically insulative and thermally conductive medium between the solid dielectric spacers, alternatives could include a solid, liquid or gas dielectric. Of importance is that the medium is electrically non-conductive but is thermally conductive and malleable so as to allow movement due to thermal expansion and contraction of the electron beam tube.